<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script src="vue.js" type="text/javascript" charset="utf-8"></script>
	</head>
	<body>
		<div id="app">
			{{a}}
			{{b}}
		</div>

		<script type="text/javascript">
			var data = {
				a: 1,
				b: 2
			};
			var vm = new Vue({
				el: "#app",
				data: data
			});

			vm.$watch('a', function(newVal, oldVal) {
				console.log(newVal, oldVal);
			})

			// vm.$data.a = "test...."
		</script>

	</body>
</html>
